This invention relates generally to wind turbines and more particularly to a vibration load reduction system for a wind turbine.
During operation, wind turbines are subjected to two types of loading. One of them is the temporary loading that occurs during an extreme gust of wind acting on the wind turbine rotor blades or an extreme wave breaking in offshore sites. The second type of loading is the relatively lower amplitude fatigue loading that repeats itself during a much longer period. This type of loading occurs during general wind turbulence and regular wave breaking in offshore situations. Current measures to counteract the extreme loading situation include turning the pitch angle to a feathered position and ensuring the tower design is adequate by properly selecting the material and reinforcing the structure. In addition a vibration opposer could help mitigate the extreme loading as well as reduce fatigue-loadings.
The dynamic loading on the wind turbine is the vibration in the tower induced by the single extreme peak load or periodic excitation force from wind or wave. Generally, designing a stiff tower is the simple and safe solution to counteract the loads. However, when the height exceeds 60–70 meters, this solution is not always practical because of the huge mass of material needed. Thus, the stiffness should be kept as low as is feasible, which leads to a “soft” tower design and the potential for more vibration.
One solution for reducing the vibration loading in a tower of a wind turbine is discussed in WO 00/77394, which uses a square box partially filled with liquid for damping oscillations of the first natural bending frequency of the wind turbine. The box provides for uni-directional damping of vibrations in the tower of the wind turbine along the two straight sides. However, because the vibrations on the tower can be multi-directional, this solution is not as effective against the vibrations on the tower from all of the directions.